Olfactory stimuli often elicit certain responses in humans. For example, certain olfactory stimuli generally make us feel refreshed while others make us feel nauseated. Yet other olfactory stimuli increase our appetite. In other words, olfactory stimuli have a wide variety of roles and effects in our daily lives. Such a variety of effects is reflected in languages such as in Japanese in which three different characters respectively express "aroma," "desirable scent" and "undesirable odor." However, despite the wide variety of scents that humans perceive, according to one theory, there are only 20 to 30 original scents, and the wide variety is generated by the combinations of these original scents.
To control the variety of undesirable odor, various products have been available. For example, deodorants and perfumes overpower body odors while air refreshers control undesirable odors. These products generally suppresses an existing odor by an overwhelming additional odor. In contrast to the above described suppression, certain other products such as bathroom products neutralize an existing undesirable odor. For example, a certain solid product is sublimated for reacting with an odor-causing agent such as ammonia in the air so as to neutralize it.
In contrast, desirable scents are used for elevating mood or promoting ambience. In fact, the use of certain aroma-causing agents has been known in aroma therapy for relaxation of body and mind. Olfactory stimuli are generated by incense burning or releasing aroma-causing agents in the air. In this regard, "Scent and Environment" Iwasaki, (Science and Engineering Books Kabushiki Kaisha, Feb. 25, 1997) discloses an aroma dispensing system which is incorporated into an air conditioning system. The aroma dispensing system circulates a selected aroma-causing agent through air ducts based upon a control signal. However, the control signal is limited to the relatively constant generation of a selected scent.
The above described scent generation system is generally limited in its capability to quickly adapt to a new circumstance. In other words, the above system is substantially static and lacks dynamic adaptation over a short period of time. Thus, it is desired that a scent generation system is able to respond to control signals each of which specifies one of a variety of distinctive olfactory stimuli. A selected scent should be presented without being affected by an existing or residual scent in the environment. Furthermore, it is also desired that the control signals are generated in response to certain conditions of the environment.